Photocatalyst mede of metal oxynitride having responsive to visible light

ABSTRACT

A photo-catalyst comprising of oxynitride of at least one transition metal and oxynitride of transition metal further containing at least one element selected from the group consisting of alkali, alkali earth and IIIB group. Especially, the transition metal contained in said photo-catalyst comprising of oxynitride of transition metal is selected from the group consisting of La, Ta, Nb, Ti and Zr, and further containing metal is selected from the group consisting of Ca, Sr, Ba, Na, K and Rb. Desirably, a promoter comprising Pt or Ni is loaded on said oxynitride of transition metal.

FIELD OF THE INVENTION

[0001] This invention relates to a novel photo catalyst, especiallyrelates to a photo-catalyst being active to visible-light comprisingoxynitride containing at least one transition metal, and aphoto-catalyst used for the water-splitting.

BACKGROUND OF THE INVENTION

[0002] The following photo-catalytic reaction is well-known as atechnique to obtain an aimed subject. That is, the light is irradiatedto a solid compound which has a photo-catalytic function so as togenerate excited electrons or holes. Then a substance is oxidized orreduced by said excited electrons or holes and obtain the aimed subject.

[0003] Especially, the photocatalytic decomposition reaction of water isbroadly interested from the view point of photo energy convertion.While, a photo-catalyst which shows activity to the photocatalyticdecomposition reaction of water can be considered to be a high qualityphoto functional material possessing functions such as photo absorption,electric charge separation or surface oxidation-reduction reaction.

[0004] Kudo, Kato et al are explaining that alkali tantalats are thephotocatalyst showing high activity to the stoichiometric photocatalyticdecomposing reaction of water by quoting various prior arts [Catal.Lett., 58(1999). 153-155, Chem. Lett., (1999), 1207, Surface Vol.36,No.12(1998), 625-645 (shortened to document A)]. In above mentioneddocument A, there is an explanation about an useful photo-catalyticmaterials for proceeding the decomposing reaction of water to hydrogenand/or oxygen using a photo-catalyst, and many indications aboutphoto-catalyst used for stoichiometric photocatalytic decomposingreaction of water are mentioned.

[0005] Further, they are referring to the photo-catalyst which carryinga promoter such as platinum or NiO.

[0006] However, the photo-catalysts explained in these documents aremainly the compound containing oxygen as a non-metallic element. And,since the band gap energy of various solid photo-catalyst is larger than3 eV, it is difficult to activate it by low energy under 3 eV. The bandgap energy can be explained as the width of a forbidden band existsbetween a valence electron band and a conduction band. On the contrary,almost all of the conventional solid photo-catalyst which can generateelectrons or holes by visible-light radiation are unstable under thecondition of photocatalytic water decomposing reaction. For example, theband gap energy of CdS or Cu—ZnS are 2.4 eV, but the catalytic reactionis restricted because it is affected by photo-corrosive action, which iscorrosive oxidative action.

[0007] Almost all of the sun light which reaches to the surface of theearth is the visible light radiation of lower energy. Therefore, for thepurpose to progress various photocatalytic reactions effectively, aphoto-catalyst which acts by visible light and is the corrosionresistance is necessary. However, among the conventional technique,there is no technique to satisfy the above mentioned requirement.

[0008] In the meanwhile, as mentioned above, almost all of the sunlightwhich can use at the surface of the earth is the visible light radiationand the object of the present invention is to provide a photo-catalystwhich can generate an excitation electron or a hole by visible lightradiation and is stable under various oxidative and reductive reactions.

[0009] Almost all of conventional stable photo-catalysts are containingoxygen as a non-metallic element. In cases of these compounds, therelative energy gap of a valence band and a conduction band is largelycontrolled by the energy level of oxygen, energy of O 2p orbit,therefore, the band gap energy is small and can not generatephoto-catalytic function by visible light radiation. The inventors ofthe present invention have noticed that a novel photo-catalyst whichacts by visible light radiation will be able to be developed if thefollowing compounds with higher valence energy level than O₂ isdeveloped. That is, the compound that when an element whose valenceelectron's energy is higher than that of oxygen is bonded with a metaland hybridize these valence electron orbits, an energy level of thevalence energy band is elevated and can reduce the band gap energy andis stable under the photo-catalytic reactive condition.

[0010] Since the valence electron of nitrogen atom has higher energythan that of an oxygen atom, the band gap energy of a metal compoundcontaining a nitrogen atom can be made smaller than that of metal oxide.The inventors of the present invention have conjectured that metal andmetal compound bonded with suitable amount of nitrogen atoms becomepossible to generate excitations electron and holes and will become aphotocatalyst which acts by visible light irradiation. And haveintensive study to find out the compound which is stable under thereacting condition of pbotocatalytic decomposition of water. Then theinventors of the present invention have found that the compoundcomprising oxynitride containing at least one transition metal acts as aphoto-catalyst and have dissolved above mentioned problem. Manycompounds among these compounds form perovskite structure. The stabilityof said compounds in photo-catalytic reaction is considered to beeffected by said crystalline structure.

DISCLOSURE OF THE INVENTION

[0011] The present invention is a photo-catalyst comprising oxynitridecontaining at least one transition metal. Desirably, the presentinvention is the photo-catalyst comprising oxynitride, wherein thetransition metal is at least one selected from the group consisting ofLa, Ta, Nb, Ti and Zr. More desirably, the present invention is thephoto-catalyst comprising oxynitride further containing at least oneelement selected from the group consisting of metal element belonging toalkali, alkali earth and IIIB group. Further desirably, the presentinvention is the photo-catalyst comprising oxynitride, wherein saidmetal element is at least one selected from the group consisting of Ca,Ba, Na, K and Rb. Still further desirably, the present invention is thephoto-catalyst, characterizing a promoter made of transition metal isloaded on said oxynitrides. Furthermore desirably, the present inventionis the photo-catalyst comprising oxynitride, wherein the promoter is Pt.

[0012] The second important point of the present invention is thephotocatalyst using said oxynitride photo-catalyst for the hydrogenevolving reaction by reduction of water or for the oxygen evolvingreaction by oxidation of water, namely, is the catalyst forphotocatalytic decomposition of water.

BRIEF ILLUSTRATION OF DRAWINGS

[0013]FIG. 1 is a graph showing the X-ray diffraction pattern of LaTaON₂photo-catalyst.

[0014]FIG. 2 is a graph showing the UV-visible diffuse reflectancespectrum of LaTaON₂ photo-catalyst.

[0015]FIG. 3 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 3wt % Pt loaded LaTaON₂ as a photo-catalyst.

[0016]FIG. 4 is a graph showing oxygen evolution rate from 0.01mol. dm⁻¹AgNO₃ aqueous solution at irradiation of the visible light longer than420 nm using 3 wt % Pt loaded LaTaON₂ as a photo-catalyst.

[0017]FIG. 5 is a graph showing the X-ray diffraction pattern of CaTaO₂Nphoto-catalyst.

[0018]FIG. 6 is a graph showing the UV-visible diffuse reflectancespectrum of CaTaO₂N photo-catalyst.

[0019]FIG. 7 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 3wt % Pt loaded CaTaO₂N as a photo-catalyst.

[0020]FIG. 8 is a graph showing oxygen evolution rate from 0.01 mol.dm⁻¹ AgNO₃ aqueous solution at irradiation of the visible light longerthan 420 nm using 3 wt % Pt loaded CaTaO₂N as a photo-catalyst.

[0021]FIG. 9 is a graph showing the X-ray diffraction pattern of SrTaO₂Nphoto-catalyst.

[0022]FIG. 10 is a graph showing the UV-visible diffuse reflectancespectrum of SrTaON₂ photo-catalyst.

[0023]FIG. 11 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 3wt % Pt loaded SrTaO₂N as a photo-catalyst.

[0024]FIG. 12 is a graph showing oxygen evolution rate from 0.01 mol.dm⁻³ AgNO₃ aqueous solution at irradiation of the visible light longerthan 420 nm using 3 wt % Pt loaded SrTaO₂N as a photo-catalyst.

[0025]FIG. 13 is a graph showing the X-ray diffraction pattern ofBaTaO₂N photo-catalyst.

[0026]FIG. 14 is a graph showing the UV-visible diffuse reflectancespectrum of BaTaO₂N photo-catalyst.

[0027]FIG. 15 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 3wt % Pt loaded BaTaO₂N as a photo-catalyst.

[0028]FIG. 16 is a graph showing oxygen evolution rate from 0.01 mol.dm⁻³ AgNO₃ aqueous solution at irradiation of the visible light longerthan 420 nm using 3 wt % Pt loaded BaTaO₂N as a photo-catalyst.

[0029]FIG. 17 is a graph showing the X-ray diffraction pattern ofCaNbO₂N photo-catalyst.

[0030]FIG. 18 is a graph showing the UV-visible diffuse reflectancespectrum of CaNbO₂N photo-catalyst.

[0031]FIG. 19 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 3wt % Pt loaded CaNbO₂N as a photo-catalyst.

[0032]FIG. 20 is a graph showing oxygen evolution rate from 0.01 mol.dm⁻¹ AgNO₃ aqueous solution at irradiation of the visible light longerthan 420 nm using 3 wt % Pt loaded CaNbO₂N as a photo-catalyst.

[0033]FIG. 21 is a graph showing the X-ray diffraction pattern ofLaTiO₂N photo-catalyst.

[0034]FIG. 22 is a graph showing the UV-visible diffuse reflectancespectrum of LaTiO₂N photo-catalyst.

[0035]FIG. 23 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 7wt % Pt loaded LaTiO₂N as a photo-catalyst.

[0036] FIG. 24 is a graph showing oxygen evolution rate from 0.01 mol.dm⁻¹ AgNO₃ aqueous solution at irradiation of the visible light longerthan 420 nm using 7 wt % Pt loaded LaTiO₂N as a photo-catalyst.

[0037]FIG. 25 is a graph showing the X-ray diffraction pattern of TaONphoto-catalyst.

[0038]FIG. 26 is a graph showing the UV-visible diffuse reflectancespectrum of TaON photo-catalyst.

[0039]FIG. 27 is a graph showing hydrogen evolution rate byphotocatalytic decomposition of water solution containing 10 vol %methanol at irradiation of the visible light longer than 420 nm using 3wt % Pt loaded TaON as a photo-catalyst.

[0040]FIG. 28 is a graph showing oxygen evolution rate from 0.01 mol.dm⁻¹ AgNO₃ aqueous solution at irradiation of the visible light longerthan 420 nm using 3 wt % Pt loaded TaON as a photo-catalyst.

THE BEST EMBODIMENT TO CARRY OUT THE INVENTION

[0041] The present invention will be illustrated more in details. Thephoto-catalyst of the present invention can be obtained by preparing ametal compound containing nitrogen atom which has absorption in visiblelight range by reacting a metal compound with a nitrogen containingcompound. As the metal compound to be a starting material, metal oxide,metallic salt or metal complex can be used And the compound havingphoto-catalytic function of the present invention can be synthesized byreacting above mentioned compound alone or the mixture of thosementioned compounds with a nitrogen containing compound such as ammonia,ammonium salt, hydrazine, metal nitride, metal amide or metallic aminecomplex.

[0042] Especially, the synthetic method of reacting between metal oxidewith ammonia is advantageous as the method for synthesizing thephoto-catalyst of the present invention. In this method, ammonia acts asa reducing agent and also as a nitriding reagent.

[0043] The supplying rate of ammonia depends on the reactingtemperature. That is, when the reacting temperature is high, thesupplying rate increases. The suitable reacting temperature is from 400°C. to 1200° C.

EXAMPLES Example 1

[0044] 9.33 g of tantalum chloride TaCl₃ and 150 g of methanol are mixedand dissolved, then 150 g of ethylene glycol and 80.00 g of citric acidare added and dissolved completely at the room temperature. 11.28 g oflanthanum nitrate 6 hydrate La(NO₃)₃. 6H₂O is added and dissolved at230° C. with constant stirring. Further, after treated by heat at 400°C. and carbonized, heat treated at 700° C. for 2 hours in theatmospheric condition, and a complex oxide precursor of La and Ta isobtained. The obtained precursor is heated to 900° C. by 1° C./mintemperature elevating speed under 1 dm³/min ammonia gas flow rate. Afterreached to above mentioned temperature, above mentioned temperature ismaintained for 20 hours, and then cooled down rapidly to the roomtemperature under He gas flow, thus oxynitride containing La and Ta issynthesized.

[0045] Pt, which is a promoter, is loaded on above mentioned oxynitridecontaining La and Ta by following process. Namely,tetraamminedichloroplatinum [Pt(NH₃)₄Cl₂] is impregnated into saidoxynitride containing La and Ta on a water bath and reduced by hydrogenat 300° C. for 2 hours. The impregnation amount of the promoter can bechanged in the limits from 0.1 to 5% by weight.

[0046] X-ray diffraction of said loaded material after calcined is shownin FIG. 1. All diffraction peaks in FIG. 1 are belonging to LaTaON₂, andthe generation of LaTaON₂ of perovskite structure is confirmed. TheUV-visible diffuse reflectance spectrum of above mentioned material isshown in FIG. 2. As clearly understood from FIG. 2, above mentionedloaded material absorbs visible light infra 650 nm.

[0047] In FIG. 3, the change by time lapse of amount of hydrogenevolution is shown, when 0.2 g of 3 wt % Pt loaded material is suspendedin 0.200 dm³ of 10 vol % methanol aqueous solution and the visible lightlonger than 420 nm is irradiated. A xenon lamp of 300 w is used as asource of light, and visible light longer than 420 nm is irradiated byusing a filter which cut the light of wave length shorter than 420 nm.As indicated in FIG. 3, it becomes clear that hydrogen can be constantlyevolved from methanol aqueous solution using above mentioned loadedmaterial under irradiation of visible light longer than 420 nm. And, inFIG. 4, the change by time lapse of amount of oxygen evolution is shown,when 0.2 g of above mentioned promoter loaded material is suspended in0.200 dm³ of 0.01 mol. dm⁻³ AgNO₃ aqueous solution and the visible lightlonger than 420 nm is irradiated. By FIG. 4, it becomes clear thatoxygen can be evolved from silver nitrate aqueous solution using abovementioned loaded material under irradiation of visible light longer than420 nm.

[0048] From above mentioned facts, it is confirmed that LaTaON₂ has aphotocatalytic activity to reduce a proton to hydrogen and oxidize waterto oxygen under the irradiation of visible light longer than 420 nm.

[0049] In the meanwhile, the material is prepared by loading Pt promoteron complex oxide of La and Ta, namely LaTaO₄, which is not treated inammonia gas flow. Said material is tested under visible lightirradiation by same reacting condition to above mentioned Pt loadedphoto-catalyst whose part of oxygen of LaTaO₄ is partially replaced bynitrogen. In this case, both hydrogen and oxygen are not evolved.

[0050] From these facts, the generation of compound havingphotocatalytic activity to visible light and having a original structuremaintaining original stability of before replacement at thephotocatalytic reacting condition by replacing a part of oxygen ofLaTaO₄ with nitrogen is conjectured.

[0051] N containing amount in above mentioned compound can be slightlyaltered, including stoichiometric ratio of LaTaON₂.

EXAMPLE 2

[0052] 12.93 g of tantalum chloride TaCl₅ and 150 g of methanol CH₈OHare mixed and dissolved, then 150 g of ethylene glycol HOCH₂CH₂OH and80.00 g of citric acid HCOOHCH₂C(OH)CH₂COOH are added and dissolvedcompletely at the room temperature. 3.61 g of calcium carbonate CaCO, isadded and dissolved at 130° C. with constant stirring. Further, aftertreated by heat at 350° C. and carbonized, heat treated at 650° C. for 2hours in the atmospheric condition, and a complex oxide precursor of Caand Ta is obtained. The obtained precursor is heated to 850° C. by 1°C./min temperature elevating speed under 1 dm³/min ammonia NH₃ gas flowrate. After reached to the said temperature, the said temperature ismaintained for 20 hours, and then cooled down rapidly to the roomtemperature under He gas flow, thus oxynitride containing Ca and Ta issynthesized. Pt, which is a promoter, is loaded on above mentionedoxynitride containing Ca and Ta by following process. Namely,tetraamminedichloroplatinum Pt(NH₃)₄Cl₂ is impregnated into saidoxynitride containing Ca and Ta on a water bath and is loaded byhydrogen reduction at 300° C. for 2 hours. The impregnation amount ofthe promoter can be changed in the limits from 0.1 to 5 wt %. X-raydiffraction of said loaded material after calcined is shown in FIG. 5.All diffraction peaks in FIG. 5 are belonging to CaTaO₂N, and thegeneration of CaTaO₂N is confirmed. The UV-visible diffuse reflectancespectrum of above mentioned material is shown in FIG. 6. As clearlyunderstood from FIG. 6, above mentioned material absorbs visible lightinfra 570 nm.

[0053] In FIG. 7, the change by time lapse of amount of hydrogenevolution is shown, when 0.2 g of 3 wt % Pt loaded material is suspendedin 0.200 dm³ of 10 vol % methanol aqueous solution and the visible lightlonger than 420 nm is irradiated. A xenon lamp of 300 w is used as asource of light and visible light longer than 420 nm is irradiated byusing a filter which cut the light of wave length shorter than 420 nm.As indicated in FIG. 7, it becomes clear that hydrogen can be constantlyevolved from methanol aqueous solution using above mentioned Pt loadedmaterial under irradiation of visible light longer than 420 nm. And, inFIG. 8, the change by time lapse of amount of oxygen evolution is shown,when 0.2 g of above mentioned Pt loaded material is suspended in 0.200dm³ of 0.01moldm⁻³AgNO₃ aqueous solution and the visible light longerthan 420 nm is irradiated. By FIG. 8, it becomes clear that oxygen canbe evolved from silver nitrate aqueous solution using above mentionedmaterial under irradiation of visible light longer than 420 nm.

[0054] From above mentioned facts, it is confirmed that CaTaO₂N has aphotocatalytic activity to reduce a proton to hydrogen and oxidize waterto oxygen under the irradiation of visible light longer than 420 nm.

[0055] In the meanwhile, the material is prepared by loading Pt promoteron complex oxide of Ca and Ta, namely Ca₂Ta₂O₇, which is not treated inammonia gas flow. Said prepared material is tested under visible lightradiation by same reacting condition to above mentioned Pt loadedphotocatalyst whose part of oxygen of Ca₂Ta₂O₇ is partially replaced bynitrogen. In this case, both hydrogen and oxygen are not evolved.

[0056] From these facts, the generation of compound havingphotocatalytic activity to visible light and having a original structuremaintaining original stability of before replacement at thephotocatalytic reacting condition by replacing a part of oxygen ofCa₂Ta₂O₇ with nitrogen is conjectured.

[0057] N containing amount in above mentioned compound can be slightlyaltered, including stoichiometric ratio of CaTaO₂N.

EXAMPLE 3

[0058] 11.04 g of tantalum chloride TaCl₅ and 150 g of methanol CH₃OHare mixed and dissolved, then 150 g of ethylene glycol HOCH₂CH₂OH and80.00 g of citric acid HCOOHCH₂C(OH)CH₂COOH are added and dissolvedcompletely at the room temperature. 4.55 g of strontium carbonate SrCO₃is added and dissolved at 130° C. with constant stirring. Further, aftertreated by heat at 350° C. and carbonized, heat treated at 650° C. for 2hours in the atmospheric condition, and a complex oxide precursor of Srand Ta is obtained. The obtained precursor is heated to 850° C. by 1° C./min temperature elevating speed under 1 dm³/min ammonia NH₃ gas flowrate. After reached to said temperature, said temperature is maintainedfor 20 hours, and then rapidly cooled down to the room temperature underHe gas flow, thus oxynitride containing Sr and Ta is synthesized. Pt,which is a promoter, is loaded on above mentioned oxynitride containingSr and Ta by following process. Namely, tetraamminedichloroplatinumPt(NH₃)₄Cl₂ is impregnated into oxynitride containing Sr and Ta on waterbath and is loaded by hydrogen reduction at 300° C. for 2 hours. Theimpregnation amount of the promoter can be changed in the limits from0.1 to 5 wt %.

[0059] X-ray diffraction of Pt loaded material after calcined is shownin FIG. 9. All diffraction peaks in FIG. 9 are belonging to SrTaO₂N, andthe generation of SrTaO₂N is confirmed. The Lw-visible diffusereflectance spectrum of above mentioned material is shown in FIG. 10. Asclearly understood from FIG. 10, above mentioned material absorbsvisible light infra 600 nm.

[0060] In FIG. 11, the change by time lapse of amount of hydrogenevolution is shown, when 0.2 g of 3 wt % Pt loaded material is suspendedin 0.200 dm³ of 10 vol % methanol aqueous solution and the visibleevolution longer than 420 nm is irradiated. A xenon lamp of 300 w isused as a source of light, and visible light longer than 420 nm isirradiated by using a filter which cut the light of wave length shorterthan 420 nm. As indicated in FIG. 11, it becomes clear that hydrogen canbe constantly evolved from methanol aqueous solution using abovementioned Pt loaded material under irradiation of visible light longerthan 420 nm. And, in FIG. 12, the change by time lapse of amount ofoxygen evolution is shown, when 0.2 g of above mentioned Pt loadedmaterial is suspended in 0.200 dm³ of 0.01 mol. dm⁻³ AgNO₃ aqueoussolution and the visible light longer than 420 nm is irradiated. By FIG.12, it becomes clear that oxygen can be evolved from silver nitrateaqueous solution using above mentioned Pt loaded material underirradiation of visible light longer than 420 nm.

[0061] From above mentioned facts, it is confirmed that SrTaO₂N has aphotocatalytic activity to reduce a proton to hydrogen and oxidize waterto oxygen under the irradiation of visible light longer than 420 nm.

[0062] In the meanwhile, the material is prepared by loading Pt promoteron complex oxide of Sr and Ta, namely Sr₂Ta₂O₇, which is not treated inammonia gas flow. The prepared material is tested under visible lightirradiation by same reacting condition to above mentioned Pt loadedphotocatalyst whose part of oxygen of Sr₂Ta₂O₇ is partially replaced bynitrogen. In this case, both hydrogen and oxygen are not generated.

[0063] From these facts, the generation of compound havingphotocatalytic activity to visible light and having an originalstructure maintaining original stability of before replacement at thephotocatalytic reacting condition by replacing a part of oxygen ofSr₂Ta₂O₇ with nitrogen is conjectured.

[0064] N containing amount in above mentioned compound can be slightlyaltered, including stoichiometric ratio of SrTaO₂N.

EXAMPLE 4

[0065] 9.57 g of tantalum chloride TaCl₅ and 150 g of methanol CH₃OH aremixed and dissolved, then 150 g of ethylene glycol HOCH₂CH₂OH and 80.00g of citric acid HCOOHCH₂C(OH)CH₂COOH are added and dissolved completelyat the room temperature. 5.54 g of barium carbonate BaCO₃ is added anddissolved at 130° C. with constant stirring. Further, after treated byheat at 350° C. and carbonized, heat treated at 650° C. for 2 hours inthe atmospheric condition, and a complex oxide precursor of Ba and Ta isobtained. The obtained precursor is heated to 850° C. by 1° C. /mintemperature elevating speed under 1 dm³/min ammonia NH₃ gas flow rate.After reached to said temperature, said temperature is maintained for 20hours, and then cooled down rapidly to the room temperature under He gasflow, thus oxynitride containing Ba and Ta is synthesized. Pt, which isa promoter, is loaded on oxynitride containing Ba and Ta by followingprocess. Namely, tetraamminedichloroplatinum Pt(NH₃)₄Cl₂ is impregnatedinto said oxynitride containing Ba and Ta on water bath and is loaded byhydrogen reduction at 573K for 2 hours. The impregnation amount of thepromoter can be changed in the limits from 0.1 to 5 wt %.

[0066] X-ray diffraction of loaded material after calcined is shown inFIG. 13. All diffraction peaks in FIG. 13 are belonging to BaTaO₂N, andthe generation of BaTaO₂N is confirmed. The UV-visible diffusereflectance spectrum of above mentioned material is shown in FIG. 14. Asclearly understood from FIG. 14, above mentioned material absorbsvisible light infra 600 nm.

[0067] In FIG. 15, the change by lapse of amount of hydrogen evolutionis shown, when 0.2 g of 3 wt % Pt loaded material is suspended in 0.200dm³ of 10 vol % methanol aqueous solution and the visible light longerthan 420 nm is irradiated. A xenon lamp of 300w is used as a source oflight and visible light longer than 420 nm is irradiated by using afilter which cut the light of wave length shorter than 420 nm. Asindicated in FIG. 15, it becomes clear that hydrogen can be constantlyevolved from methanol aqueous solution using above mentioned Pt loadedmaterial under irradiation of visible light longer than 420 nm. And, inFIG. 16, the change by time lapse of amount of oxygen evolution isshown, when 0.2 g of above mentioned Pt loaded material is suspended in0.200 dm³ of 0.01 moldm⁻³AgNO₃ aqueous solution and the visible lightlonger than 420 nm is irradiated. By FIG. 16, it becomes clear thatoxygen can be evolved from silver nitrate aqueous solution using abovementioned Pt loaded material under irradiation of visible light longerthan 420 nm.

[0068] From above mentioned facts, it is confirmed that BaTaO₂N has aphotocatalytic activity to reduce a proton to hydrogen and oxidize waterto oxygen under the irradiation of visible light longer than 420 nm.

[0069] In the meanwhile, the material is prepared by loading Pt promoteron complex oxide of Ba and Ta, namely Ba₂Ta₂O₇, which is not treated inammonia gas flow. Said material is tested under visible light by samereacting condition to above mentioned Pt loaded photo-catalyst whosepart of oxygen of Ba₂Ta₂O₇ is partially replaced by nitrogen. In thiscase, both hydrogen and oxygen are not generated.

[0070] From these facts, the generation of compound havingphotocatalytic activity to visible light and having an originalstructure maintaining original stability of before replacement at thephotocatalytic reacting condition by replacing a part of oxygen ofBa₂Ta₂O₇ with nitrogen is conjectured.

[0071] N containing amount in above mentioned compound can be slightlyaltered, including stoichiometric ratio of BaTaO₂N.

EXAMPLE 5

[0072] 14.29 g of niobium chloride NbCl₅ and 150 g of methanol CH₃OH aremixed and dissolved, then 150 g of ethylene glycol HOCH₂CH₂OH and 80.00g of citric acid HCOOHCH₂C(OH)CH₂COOH are added and dissolved completelyat the room temperature. 5.30 g of calcium carbonate CaCO₃ is added anddissolved at 130° C. with constant stirring. Further, after treated byheat at 350° C. and carbonized, heat treated at 650° C. for 2 hours inthe atmospheric condition, and a complex oxide precursor of Ca and Nb isobtained. The obtained precursor is heated to 850° C. by 1° C. /mintemperature elevating speed under 1 dm³/min ammonia NH₃ gas flow rate.After reached to said temperature, said temperature is maintained for 20hours, and then rapidly cooled down to the room temperature under He gasflow, thus oxynitride containing Ca and Nb is synthesized.

[0073] Pt, which is a promoter, is loaded on oxynitride containing Caand Nb by following process. Namely, tetraamminedichloroplatinumPt(NH₃)₄Cl₂ is impregnated into oxynitride containing Ca and Nb on waterbath and is loaded by hydrogen reduction at 300° C. for 2 hours. Theimpregnation amount of the promoter can be changed in the limits from0.1 to 5 wt %.

[0074] X-ray diffraction of Pt loaded material after calcined is shownin FIG. 17. All diffraction peaks in FIG. 17 are belonging to CaNbO₂N,and the generation of CaNbO₂N is confirmed. The UV-visible diffusereflectance spectrum of above mentioned Pt loaded material is shown inFIG. 18. As clearly understood from FIG. 18, above mentioned materialabsorbs visible light infra 630 nm.

[0075] In FIG. 19, the change by lapse of amount of hydrogen evolutionis shown, when 0.2 g of 3 wt % Pt loaded material is suspended in 0.200dm³ of 10 vol % methanol aqueous solution and the visible light longerthan 420 nm is irradiated.

[0076] A xenon lamp of 300 w is used as a source of light, and visiblelight longer than 420 nm is irradiated by using a filter which cut thelight of wave length shorter than 420 nm. As indicated in FIG. 19, itbecomes clear that hydrogen can be constantly evolved from methanolaqueous solution using above mentioned material under irradiation ofvisible light longer than 420 nm. And, in FIG. 20, the change by timelapse of amount of oxygen evolution is shown, when 0.2 g of abovementioned Pt loaded material is suspended in 0.200 dm³ of 0.01 mol. dm⁻¹AgNO₃ aqueous solution and the visible light longer than 420 nm isirradiated. By FIG. 20, it becomes clear that oxygen can be generatedfrom silver nitrate aqueous solution using above mentioned Pt loadedmaterial under irradiation of visible light longer than 420 nm.

[0077] From above mentioned facts, it is confirmed that CaNbO₂N has aphotocatalytic activity to reduce a proton to hydrogen and oxidize waterto oxygen under the irradiation of visible light longer than 420 nm.

[0078] In the meanwhile, the material is prepared by loading Pt promoteron complex oxide of Ca and Nb, namely Ca₂Nb₂O₇, which is not treated inammonia gas flow. Said prepared material is tested under visible lightby same reacting condition to above mentioned Pt loaded photo-catalystwhose part of oxygen of Ca₂Nb₂O₇ is partially replaced by nitrogen. Inthis case, both hydrogen and oxygen are not evolved.

[0079] From these facts, the generation of compound havingphotocatalytic activity to visible light and having a original structuremaintaining original stability of before replacement at thephotocatalytic reacting condition by replacing a part of oxygen ofCa₂Nb₂O₇ with nitrogen is conjectured.

[0080] N containing amount in above mentioned compound can be slightlyaltered, including stoichiometric number ratio of CaNbO₂N.

EXAMPLE 6

[0081] 5.68 g of titaniumisopropoxide Ti[OCH(CH₃)₂]₄ and 98.81 g ofethylene glycol HOCH₂CH₂OH are mixed together, then 8.66 g of lanthanumnitrate 6hydrate La(NO₃)₃. 6H₂O is added and dissolved at roomtemperature with constant stirring. 76.49 g of citric acidHCOOHCH₂C(OH)CH₂COOH and 102.0 g of methanol CH₃OH are added anddissolved at 130° C. with constant stirring. Further, after treated byheat at 350° C. and carbonized, heat treated at 650° C. for 2 hours inthe atmospheric condition, and a complex oxide precursor of La and Ti isobtained. The obtained precursor is heated to 950° C. by 1° C./mintemperature elevating speed under 1 dm³/min ammonia NH₃ gas flow rate.After reached to the said temperature, said temperature is maintainedfor 15 hours, and then cooled down rapidly to the room temperature underAr gas flow, thus oxynitride containing Ti and La is synthesized. Pt,which is a promoter, is loaded on oxynitride containing Ti and La byfollowing process. Namely, 0.00357dm³ (Pt 7 wt %) of 0.1moldm⁻³tetraamminedichloroplatinum Pt(NH₃)₄Cl₂ solution is impregnated into 0.3g of said oxynitride containing Ti and La on a water bath so as toevaporate water and is loaded by hydrogen reduction at 300° C. for 2hours.

[0082] X-ray diffraction of loaded material after calcined is shown inFIG. 21. All diffraction peaks in FIG. 21 are belonging to LaTiO₂N, andthe generation of laTiO₂N is confirmed. The UV-visible diffusereflectance spectrum of above mentioned material is shown in FIG. 22. Asclearly understood from FIG. 22, above mentioned loaded material absorbsvisible light infra 600 nm.

[0083] In FIG. 23, the change by lapse of amount of hydrogen evolutionis shown, when 0.2 g of 7 wt % Pt loaded material is suspended in 0.310dm³ of 10 vol % methanol aqueous solution and the visible light longerthan 400 nm is irradiated. A high voltage mercury lamp of 450w is usedas a source of light, and visible light longer than 400 nm is irradiatedby using a filter of sodium nitrite aqueous solution.

[0084] As indicated in FIG. 23, it becomes clear that hydrogen can beconstantly generated from methanol aqueous solution using abovementioned material under irradiation of visible light longer than 400nm. And, in FIG. 24, the change by time lapse of amount of oxygenevolution is shown, when 0.2 g of above mentioned Pt loaded material issuspended in 0.310 dm³ of 0.01 mol. dm⁻³ AgNO₃ aqueous solution and thevisible light longer than 400 nm is irradiated. By FIG. 24, it becomesclear that oxygen can be generated from silver nitrate aqueous solutionusing above mentioned material under irradiation of visible light longerthan 400 nm.

[0085] From above mentioned facts, it is confirmed that LaTiO₂N has anability to reduce a proton to hydrogen and oxidize water to oxygen underthe irradiation of visible light radiation longer than 400 nm.

[0086] In the meanwhile, the material is prepared by loading platinumpromoter on complex oxide of La and Ti which is not treated in ammoniagas flow. The prepared material is tested under visible lightirradiation by same reacting condition to above mentioned Pt loadedphoto-catalyst whose part of oxygen of La₂Ti₂O₇ is partially replaced bynitrogen. In this case, both hydrogen and oxygen are not evolved.

[0087] From these facts, the generation of compound having photoactivity to visible light and having an original structure maintainingoriginal stability of before replacement at the photocatalytic reactingcondition by replacing a part of oxygen of La₂Ti₂O₇ with nitrogen isconjectured.

[0088] N containing amount in above mentioned compound can be slightlyaltered, including stoichiometric ratio of LaTiO₂N.

EXAMPLE 7

[0089] 5.00 g of tantalum oxide is heated to 850° C. by 1° C./mintemperature elevating speed under 20 cm³/min ammonia NH₃ gas flow rateand maintained said temperature for 20 hours. Then cooled down rapidlyto the room temperature under HM₃ gas flow, and oxynitride containing Tais synthesized.

[0090] Pt, which is a promoter, is loaded on above mentioned oxynitridecontaining Ta material by following process. Namely,tetraamminedichloroplatinum Pt(NH₃)₄Cl₂ is impregnated into saidoxynitride containing Ta material on water bath and is loaded byhydrogen reduction at 300° C. for 2 hours. The impregnation amount ofthe promoter can be changed in the limits from 0.1 to 5 wt %.

[0091] X-ray diffraction of said Pt loaded material after calcined isshown in FIG. 25. All diffraction peaks in FIG. 25 are belonging toTaON, and the generation of TaON is confirmed. The UV-visible diffusereflectance spectrum of above mentioned Pt loaded material is shown inFIG. 26. As clearly understood from FIG. 26, above mentioned Pt loadedmaterial absorbs light radiation infra 550 nm.

[0092] In FIG. 27, the change by lapse of time of amount of hydrogenevolution is shown, when 0.2 g of 3 wt % platinum loaded material issuspended in 0.200 dm³ of 10 vol % methanol aqueous solution and thevisible light longer than 420 nm is irradiated.

[0093] A xenon lamp of 300 w is used as a source of light, and visiblelight longer than 420 nm is irradiated by using a filter which cut thelight of wave length shorter than 420 nm. As indicated in FIG. 27, itbecomes clear that hydrogen can be constantly evolved from methanolaqueous solution using above mentioned loaded material under irradiationof visible light longer than 420 nm. And, in FIG. 28, the change by timelapse of amount of oxygen evolution is shown, when 0.2 g of abovementioned loaded material is suspended in 0.200 dm³ of 0.01 mol. dm⁻³AgNO₃ aqueous solution and the visible light longer than 420 nm isirradiated. By FIG. 28, it becomes clear that oxygen can be evolved fromsilver nitrate aqueous solution using above mentioned Pt loaded materialunder irradiation of visible light longer than 420 nm.

[0094] From above mentioned facts, it is confirmed that TaON has aphotocatalytic activity to reduce a proton to hydrogen and oxidize waterto oxygen under the irradiation of visible light longer than 420 nm.

[0095] In the meanwhile, a material is prepared by loading platinumpromoter on tantalum oxide Ta₂O₅ which is not treated in ammonia gasflow. Said material is tested under visible light by same reactingcondition to above mentioned loaded photo-catalyst whose part of oxygenof Ta₂O₅ is partially replaced by nitrogen. In this case, both hydrogenand oxygen are not evolved.

[0096] From these facts, the generation of compound havingphotocatalytic activity to visible light and having an originalstructure maintaining original stability of before replacement at thephotocatalytic reacting condition by replacing a part of oxygen of Ta₂O₅with nitrogen is conjectured.

[0097] N containing amount in above mentioned compound can be slightlyaltered, including integral number ratio of TaON.

[0098] The oxynitride in the present invention is the photo-catalystwhich can evolve hydrogen and oxygen by irradiation of visible lightradiation under the presence of an electron-donor sacrificial reagent(methanol) and an electron-acceptor sacrificial reagent (silvernitrate). This fact indicates that the oxynitride of the presentinvention has a possibility to decompose water completely under theirradiation of visible light. When an electric charge separating effectis enhanced by removing a lattice defect which can be a recombinationsite of an electron and a hole and by adding optimum hydrogen generatingpromoter and oxidation promoter of water, the oxynitride of the presentinvention can be a photo-catalyst which decomposes water completely bythe irradiation of visible radiation.

POSSIBILITY FOR THE INDUSTRIAL USE

[0099] As mentioned above, the photo-catalyst obtained by the presentinvention, is the catalyst that acts by visible radiation, which is themajority in sun light reaching to the surface of the earth. By carryingout photocatalyst reaction with sun light, the useful compound can beproduced. Further, as indicated in Examples, since said photo-catalysthas an ability to decompose water to hydrogen and oxygen by visibleradiation, it is hopeful to be used as a photo-catalyst convert sunlight to hydrogen which is considered as the energy of next generation.

What is claim:
 1. A photocatalyst comprising oxynitride containing atleast one transition metal.
 2. The photocatalyst of claim 1, wherein thetransition metal contained in the oxynitride is at least one selectedfrom the group consisting of La, Ta, Nb, Th and Zr.
 3. The photocatalystaccording to any one of claim 1 or 2, wherein the oxynitride furthercontains at least one element selected from the group consisting ofmetal elements belonging to alkali, alkali earth and IIIB group.
 4. Thephotocatalyst of claim 3, wherein the metal element contained in theoxynitride is at least one selected from the group consisting of Ca, Sr,Ba, K and Rb.
 5. The photo-catalyst according to any one of claims 1 to4, wherein a promoter comprising transition metal is loaded on theoxynitride.
 6. The photo-catalyst of claim 5, wherein the promoterloaded on the oxynitride is Pt.
 7. An use of the oxynitridephoto-catalyst according to any one of claims 1 to 6 as a catalyst forphoto decomposition of water.